Conventionally, there is available an ultrasonic sensor that includes, as shown in FIG. 13, a hollow housing 101 having an opening on one surface thereof, a transceiver device 102 having a wave transceiving surface for transmitting and receiving ultrasonic waves, the wave transceiving surface exposed on the other surface of the housing 101, a circuit board 103 arranged within the housing 101 and mounted with an electronic circuit for processing ultrasonic waves transmitted and received through the transceiver device 102, a wiring line 104 electrically interconnecting the transceiver device 102 and the circuit board 103, a cover member 105 installed to cover the opening of the housing 101, and a terminal 106 connected at one end to the circuit board 103 by welding or other methods and at the other end to a power supply terminal (not shown).
When used in a motor vehicle, the ultrasonic sensor is installed in a portion highly susceptible to drenching and severe vibrating, e.g., a bumper or a front grill. In many conventional ultrasonic sensors, therefore, a filler material 107 (such as silicone or the like) having hydrophobicity and elasticity is filled in the housing 101 accommodating the circuit board 103 and so forth. It is a typical configuration to secure an increased waterproof property and a high vibration resistance in this manner (see, e.g., Japanese Patent Application Publication No. 2005-24351).
In the conventional example stated above, however, the weight and cost of the ultrasonic sensor is proportionately increased as the filler material 107 is filled in the housing 101. In addition, there is a possibility that, due to the existence of the filler material 107, distortions are generated in the circuit board 103 arranged within the housing 101. It is also likely that cracks are generated in the solder portions between the circuit board 103 and the electronic parts mounted on the circuit board 103. There is also the likelihood that the sensing area characteristics of the sensor are changed before and after filling the filler material 107. Other causes of generating distortions in the circuit board 103 include, e.g., an external load generated by the thermal expansion and shrinkage of the filler material 107. Examples of the change in the sensing area characteristics before and after filling the filler material 107 include the narrowing of a sensing area of the sensor. There is also a problem in that a great deal of time is required in the drying process. In this viewpoint, there remains a room for improvement.